This invention relates generally to radio frequency energy (RF) systems and more particularly to systems which determine the direction from which an RF signal is received.
Direction finding systems have been employed for many purposes. One widely used technique for direction finding is called "amplitude monopulse".
A multibeam amplitude monopulse system receives a plurality of evenly spaced beams of RF energy (hereafter simply "beams"). The center of each beam is associated with a given direction. When a signal is received in one beam, the angle associated with that beam gives a coarse indication of the direction from which the signal is impinging on the antenna.
To get a finer measurement of the direction of the signal, adjacent beams overlap so that each signal falls into two beams. The relative strength of the signal in each beam indicates the angular difference between the direction of signal and the center of the beams. Thus, the direction of the signal can be precisely determined.
A direction finding system must provide receive beams covering every direction in which a signal of interest might be received. Conventional systems often must provide receive beams in all directions--what is called 360.degree. coverage. To provide 360.degree. coverage, conventional systems contain at least four array antennas. Each of the antennas covers a different sector of the 360.degree. coverage area.
One shortcoming of such an arrangement is the amount of components needed to construct the system. For example, each antenna element in each of the array antennas requires a low noise amplifier. Such amplifiers are costly.
The problem is further compounded if the direction finding system must work on signals over a relatively wide range of frequencies. Basically, the accuracy of the direction finding measurement depends on the width of the received beams in combination with the spacing between the direction of adjacent beams. The width of the receive beam decreases with increasing frequency. Thus, to have an acceptable accuracy on the direction finding measurement, the spacing between the beams must be decreased to operate the direction finding system.
To decrease the spacing between adjacent beams, the antenna array is made longer. Since the spacing between elements must be less than one-half of a wavelength to avoid grating lobes, more antenna elements are added to each array to make the array longer.
Of course, when a direction finding system contains a plurality of linear arrays, it is not possible to add single antenna elements to improve the operating bandwidth of the system. One antenna element must be added to each array, meaning at least four antenna elements are added at a time in a system which provides 360.degree. coverage.
Moreover, the gain of a linear array is proportional to the length of the array. The number of elements might need to be further increased to provide adequate gain.
An additional shortcoming of a direction finding system with linear arrays is called "coning error". Briefly, coning error results because of the geometrical interaction between the azimuth and elevation lines-of-sight at azimuth angles off broadside. Thus, the measured azimuth angle will deviate from the true azimuth angle as the elevation angle increases.